Electronic structure of pristine and sodium doped poly(p-pyridine)

The electronic structure of pristine and sodium-doped poly(p-pyridine) has been studied using both ultraviolet and x-ray photoelectron spectroscopy. The spectra are interpreted with the help of the results of quantum-chemical calculations. Electronic band-structure calculations are performed for isolated chains with different connectivity patterns (head-to-tail and head-to-head), using the valence effective Hamiltonian (VEH) method, with geometries derived from optimizations using the Austin Model 1 Hamiltonian. The density-of-valence-states are derived directly from the VEH band structure. Excellent agreement is obtained between the theoretical simulations and the experimental data, which allows for a detailed assignment of the different peaks in the spectra. The C(1s) and N(1s) shake-up spectra of poly(p-pyridine) are analyzed on the basis of corresponding data for pyridine in the gas phase. Upon sodium doping of poly(p-pyridine), new states are observed within the otherwise forbidden energy gap. These new states can be assigned to the formation of bipolarons.